Wavelength converters, including polarization-enhanced carrier capture converters, for solid state lighting devices, and associated systems and methods

ABSTRACT

Wavelength converters, including polarization-enhanced carrier capture converters, for solid state lighting devices, and associated systems and methods are disclosed. A solid state radiative semiconductor structure in accordance with a particular embodiment includes a first region having a first value of a material characteristic and being positioned to receive radiation at a first wavelength. The structure can further include a second region positioned adjacent to the first region to emit radiation at a second wavelength different than the first wavelength. The second region has a second value of the material characteristic that is different than the first value, with the first and second values of the characteristic forming a potential gradient to drive electrons, holes, or both electrons and holes in the radiative structure from the first region to the second region. In a further particular embodiment, the material characteristic includes material polarization.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/216,062, filed Aug. 23, 2011, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present technology is directed generally to wavelength converters,including polarization-enhanced carrier capture converters, for solidstate lighting devices, and associated systems and methods. Wavelengthconverters in accordance with the present technology are suitable forLEDs and other radiation emitting devices.

BACKGROUND

Solid state lighting (“SSL”) devices are used in a wide variety ofproducts and applications. For example, mobile phones, personal digitalassistants (“PDAs”), digital cameras, MP3 players, and other portableelectronic devices utilize SSL devices (e.g., LEDs) for backlighting andother purposes. SSL devices are also used for signage, indoor lighting,outdoor lighting, and other types of general illumination. FIG. 1A is across-sectional view of a conventional SSL device 10 a with lateralcontacts. As shown in FIG. 1A, the SSL device 10 a includes a substrate20 carrying an LED structure 11 having an active region 14, e.g.,containing gallium nitride/indium gallium nitride (GaN/InGaN) multiplequantum wells (“MQWs”), positioned between N-type GaN 15 and P-type GaN16. The SSL device 10 a also includes a first contact 17 on the P-typeGaN 16 and a second contact 19 on the N-type GaN 15. The first contact17 typically includes a transparent and conductive material (e.g.,indium tin oxide (“ITO”)) to allow light to escape from the LEDstructure 11. In operation, electrical power is provided to the SSLdevice 10 a via the contacts 17, 19, causing the active region 14 toemit light.

FIG. 1B is a cross-sectional view of another conventional LED device 10b in which the first and second contacts 17 and 19 are opposite eachother, e.g., in a vertical rather than lateral configuration. Duringformation of the LED device 10 b, a growth substrate, similar to thesubstrate 20 shown in FIG. 1A, initially carries an N-type GaN 15, anactive region 14 and a P-type GaN 16. The first contact 17 is disposedon the P-type GaN 16, and a carrier 21 is attached to the first contact17. The substrate 20 is removed, allowing the second contact 19 to bedisposed on the N-type GaN 15. The structure is then inverted to producethe orientation shown in FIG. 1B. In the LED device 10 b, the firstcontact 17 typically includes a reflective and conductive material(e.g., silver or aluminum) to direct light toward the N-type GaN 15.

One drawback with existing LEDs is that they do not emit white light.Instead, LEDs typically emit light within only a narrow wavelengthrange. For human eyes to perceive the color white, a broad range ofwavelengths is needed. Accordingly, one conventional technique foremulating white light with LEDs is to deposit a converter material(e.g., a phosphor) on an LED die. FIG. 1C shows a conventional SSLdevice 10 c that includes a support 2 carrying an LED die 4 and aconverter material 6. In operation, an electrical voltage is applied tothe die 4 via contacts having an arrangement generally similar to thatshown in either FIG. 1A or FIG. 1B. In response to the applied voltage,the active region of the LED die 4 produces a first emission (e.g., ablue light) that stimulates the converter material 6 to emit a secondemission (e.g., a yellow light). The combination of the first and secondemissions appears white to human eyes if matched appropriately. Asdiscussed in more detail below, using phosphor converter materials to“convert” blue light into white light has certain drawbacks.Accordingly, there is a need for light emitting devices that can producelight at a particular wavelength without phosphor converter materials.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale. Instead, emphasis is placed on clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1A is a partially schematic, cross-sectional diagram of an SSLdevice having a lateral arrangement in accordance with the prior art.

FIG. 1B is a partially schematic, cross-sectional diagram of another SSLdevice having a vertical arrangement in accordance with the prior art.

FIG. 1C is a partially schematic, cross-sectional diagram of a lightemitting device having a phosphor converter material positioned inaccordance with the prior art.

FIG. 2 is a partially schematic, cross-sectional illustration of asystem that includes a light source and a radiative structure configuredin accordance with an embodiment of the presently disclosed technology.

FIGS. 3A-3F are schematic, perspective views of various crystal planesin a GaN/InGaN material in accordance with embodiments of the presentlydisclosed technology.

FIG. 4 is a band diagram illustrating interfaces between materialshaving different material polarizations in accordance with an embodimentof the presently disclosed technology.

FIG. 5 is a partially schematic illustration of a portion of a radiativestructure having quantum wells and barriers alternating in accordancewith an embodiment of the presently disclosed technology.

FIG. 6 is a band diagram illustrating the behavior expected of arepresentative structure shown in FIG. 5, in accordance with anembodiment of the presently disclosed technology.

FIGS. 7A and 7B are band diagrams comparing band gap energies forsimulated structures having non-polar characteristics (FIG. 7A) andpolar characteristics (FIG. 7B) in accordance with embodiments of thepresently disclosed technology.

DETAILED DESCRIPTION

Embodiments of the presently disclosed technology are directed generallyto wavelength converters for solid state lighting (“SSL”) devices, andassociated systems and methods. As used hereinafter, the term “SSLdevice” generally refers to devices with light emitting diodes (“LEDs”),organic light emitting diodes (“OLEDs”), laser diodes (“LDs”), polymerlight emitting diodes (“PLEDs”), and/or other suitable sources ofillumination, other than electrical filaments, a plasma, or a gas.Briefly described, a radiation system in accordance with a particularembodiment of the disclosed technology includes a solid state radiativesemiconductor structure that has a first region and a second region. Thefirst region has a first value of a material characteristic, and thesecond region has a second value of the material characteristic that isdifferent than the first value. The first region is positioned toreceive radiation at a first wavelength, and the second region ispositioned adjacent to the first region to emit radiation at a secondwavelength different than the first wavelength. The first and secondvalues of the characteristic form a potential gradient to driveelectrons, holes, or both electrons and holes in the radiative structurefrom the first region to the second region. Accordingly, the secondregion can receive optically generated carriers from the first regionand emit radiation at the second wavelength. In particular embodiments,the radiative semiconductor structure can be positioned proximate to anenergy source that directs radiation at the first wavelength toward thefirst region of the semiconductor structure. In further particularembodiments, the energy source can include a solid state lightingdevice, for example, an LED.

Other systems, methods, features, and advantages of the presentlydisclosed technology will become apparent to one of ordinary skill inthe art. Several details describing structures or processes that arewell-known and often associated with such systems and methods, but thatmay unnecessarily obscure some significant aspects of the disclosure,are not set forth in the following description for purposes of clarity.Moreover, although the following disclosure sets forth severalembodiments of different aspects of the technology disclosed herein,several other embodiments can include different configurations ordifferent components than those described in this section. Accordingly,the disclosed technology may have other embodiments with additionalelements, and/or without several of the elements described below withreference to FIGS. 2-7.

FIG. 2 is a partially schematic, cross-sectional illustration of asystem 200 that receives or absorbs energy at one wavelength andre-radiates or emits energy at another wavelength. In a particularembodiment, the system 200 includes a support 230 carrying a lightsource 250. The light source 250 can include an LED or other SSL device,or another device (e.g., a laser) that emits first radiation at a firstwavelength A. The system 200 further includes a radiative structure 240positioned to receive and absorb the first radiation and emit secondradiation at a different wavelength E. The radiative structure 240 caninclude one or more first regions 241 (e.g., absorptive regions) and oneor more second regions 242 (e.g., emissive regions). For example, in theembodiment shown in FIG. 2, the radiative structure 240 includes twoabsorptive regions 241, shown as a first absorptive region 241 a and asecond absorptive region 241 b, positioned on opposite sides of a singleemissive region 242. As used herein, the term “absorptive region” refersgenerally to a region having suitable (e.g., strong) absorptivecharacteristics at the first wavelength A emitted by the light source250. The term “emissive region” refers generally to a region havingsuitable (e.g., strong) emissive characteristics at the secondwavelength E. In any of these embodiments, the radiative structure 240can replace conventional phosphor structures and can accordingly modifythe spectrum of light emitted by overall system 200 without the use ofphosphors, or with a reduced use of phosphors. Further features andadvantages of representative systems are described below with referenceto FIGS. 3A-7.

Particular embodiments of the presently disclosed technology aredescribed below in the context of radiative structures having differentregions with different material polarizations, resulting, for example,from a difference in material composition or strain of the materialsforming the regions and a particular crystal orientation. In otherembodiments, the material characteristics of the regions can have otherdiffering characteristics. For example, the regions can have differentcompositions that produce different bandgap energies. Particularembodiments are disclosed in co-pending U.S. application Ser. No.______, titled “Wavelength Converters for Solid State Lighting Devices,and Associated Systems and Methods” (Attorney Docket No. 10829.9046US),filed concurrently herewith and incorporated herein by reference.

FIGS. 3A-3F are schematic perspective views of various crystal planes ina portion of a GaN/InGaN material. In FIGS. 3A-3F, Ga (or Ga/In) and Natoms are schematically shown as large and small spheres, respectively.As shown in FIGS. 3A-3F, the GaN/InGaN material has a wurtzite crystalstructure with various lattice planes or facets as represented bycorresponding Miller indices. A discussion of the Miller index can befound in the Handbook of Semiconductor Silicon Technology by William C.O'Mara.

As used hereinafter, a “polar plane” generally refers to a crystal planein a crystal structure that contains only one type of atom. For example,as shown in FIG. 2A, the polar plane denoted as the “c-plane” in thewurtzite crystal structure with a Miller index of (0001) contains onlyGa atoms. Similarly, other polar planes in the wurtzite crystalstructure may contain only N atoms and/or other suitable type of atoms.

As used hereinafter, a “non-polar plane” generally refers to a crystalplane in a crystal structure that is generally perpendicular to a polarplane (e.g., to the c-plane). For example, FIG. 3B shows a non-polarplane denoted as the “a-plane” in the wurtzite crystal structure with aMiller index of (1120). FIG. 3C shows another non-polar plane denoted asthe “m-plane” in the wurtzite crystal structure with a Miller index of(1010). Both the a-plane and the m-plane are generally perpendicular tothe c-plane shown in FIG. 3A.

As used hereinafter, a “semi-polar plane” generally refers to a crystalplane in a crystal structure that is canted relative to a polar plane(e.g., to the c-plane) without being perpendicular to the polar plane.For example, as shown in FIGS. 3D-3F, each of the semi-polar planes inthe wurtzite crystal structure with Miller indices of (1013), (1011),and (1122) form an angle with the c-plane shown in FIG. 3A. The angle isgreater than 0° but less than 90°. Only particular examples of crystalplanes are illustrated in FIGS. 3A-3F. Accordingly, the polar,non-polar, and semi-polar planes can also include other crystal planesnot specifically illustrated in FIGS. 3A-3F. In general, the designercan select the material polarization of the materials forming wavelengthconverters in accordance with embodiments of the present disclosure byselecting the angle along which a corresponding epitaxial substrate iscut. This in turn determines the material polarization of the subsequentepitaxially grown layers. In general, different layers will have thesame crystal orientation (as individual layers are grown epitaxially onthe layer below), but will have different material polarizations, e.g.due to different concentrations of particular constituents, such asindium.

As described above with reference to FIG. 2, wavelength converters inaccordance with the present technology can include one or more first orabsorptive regions, and one or more second or emissive regions. Theemissive regions can receive carriers from adjacent absorptive regionsand can accordingly form quantum wells. FIG. 4 is a representative banddiagram illustrating a conduction band 445 and a valance band 446 for asecond region 242 (e.g., a quantum well) surrounded by correspondingfirst regions 241 a, 241 b. In a particular aspect of this embodiment,the second region 242 is a single, 2 nm-wide gallium indium nitridestructure grown along the c-axis. In still a further particularembodiment, the composition of the second region 242 isGa_(0.78)In_(0.22)N. In other embodiments, the second region 242 caninclude other gallium indium nitride structures, other III-nitrideheterostructures, and/or other non-Group III heterostructures (e.g.,Group II or Group VI heterostructures). In any of these embodiments, thesecond region 242 can form interfaces (e.g., heterointerfaces) 444 atthe junctions between the second region 242 and the surrounding firstregions 241 a, 241 b. The heterointerfaces 444 may be graded or abrupt,depending upon the particular embodiment. In general, based on thedifferent material polarizations in the first and second regions, theheterointerfaces create electric fields that assist in the transport ofoptically generated electron-hole pairs from the first regions 241 a,241 b to the second region 242.

In a particular aspect of an embodiment shown in FIG. 4, thediscontinuity in the polarization field between the second region 242and the adjacent first regions 241 a, 241 b creates a negative sheetcharge to the left of the second region 242. This negative sheet chargerepels electrons and pushes the edge of the conduction band 445 upwards.To the right of the second region 242 is a positive sheet charge whichattracts electrons and pulls the conduction band edge 445 down. Theelectric field to the left of the second region 242 pushes electrons tothe left, as indicated by arrow R1. In the second region 242, anelectric field pushes the electrons to the right, as indicated by arrowR2. Holes are pushed in the opposite direction: to the right in thefirst region 241 a (as indicated by arrow R3) and to the left in thesecond region (as indicated by arrow R4).

As described further below with reference to FIGS. 5-7B, radiativestructures can be designed with multiple quantum wells separated bycorresponding barriers in such a way that the polarization mismatches atthe heterointerfaces between these structures create electric fieldsthat funnel carriers (e.g., holes and/or electrons) into the adjacentlayers, so as to produce emitted light at a target wavelength.

FIG. 5 is a schematic illustration of a representative radiativestructure 540 that includes multiple quantum wells 542 (four are shownin FIG. 5), interleaved with multiple barriers 543 (four are shown inFIG. 5), and sandwiched between corresponding first regions 541 a, 541b. Embodiments of the present disclosure are not limited to those shownin FIG. 5. For example, other embodiments can include any suitablenumber of quantum wells and barriers, depending on devicecharacteristics. In particular embodiments, the first regions 541 a, 541b can include N-GaN. In the illustrated embodiment, the lower firstregion 541 a can provide a substrate for epitaxial growth of thebarriers 543 and the quantum wells 542. The upper first region 541 b canbe eliminated in some embodiments. However, an advantage of the upperfirst region 541 b is that it can separate the quantum wells 542 fromthe outermost surface of the structure 540, thus reducing or eliminatingsurface charges that can adversely affect the performance of thestructure 540. The quantum well composition can be Ga_(0.78)In_(0.22)N,and the barrier composition can be Ga_(0.9)In_(0.1)N, in particularembodiments, and can have other compositions in other embodiments. Thedoping concentration can be 5×10¹⁸ cm⁻³ in the N-GaN and in the barriers543, and can have other values in other embodiments. The quantum wells542 and corresponding barriers 543 are stacked (e.g., grown) generallyperpendicular to the c-axis, as shown in FIG. 5. In other embodiments,the barriers 543 can be N-GaN without indium. However, an advantage ofincluding indium (or another suitable element) is that it is expected toincrease the absorptivity of the barriers 543, and therefore thewavelength conversion efficiency of the structure 540. In still furtherembodiments, materials other than GaInN can be used for the quantumwells 542 and the barriers 543. Suitable materials include zinc oxide orother materials having a wurtzite crystal structure.

FIG. 6 illustrates a band diagram corresponding to the structuredescribed above with reference to FIG. 5. Each of the quantum wells 542has a well width W, of approximately 2 nanometers, and each barrier 543has a barrier width W_(B) of approximately 10 nanometers. Accordingly,the total absorption thickness (e.g., the thickness of materialabsorbing incident radiation at a first wavelength) is 48 nanometers,while the total quantum well thickness is 8 nanometers. Both thebarriers 543 and the quantum wells 542 absorb incident light. As thelight is absorbed, it generates electrons and holes (e.g., opticallygenerated carriers). When the electrons and holes are located in thequantum wells 542, they are at an electropotential minimum (or relativeminimum) and are accordingly confined. When electrons are located in thebarriers 543, they experience an electric field, which pushes theelectron toward a nearby quantum well 542. For example, in the left-mostbarrier 543 shown in FIG. 6, electrons are pushed to the right, asindicated by arrow R5. In the next three barriers 543, the electrons arepushed to the left, as indicated by arrows R6. In a generally similarmanner, holes located in a barrier are pushed toward a correspondingquantum well by the field resulting from the difference in polarizationbetween the barrier 543 and the quantum well 542. For example, holes inthe right-most three barriers 543 are forced to the right as indicatedby arrows R7 and into the adjacent quantum wells 542. Holes in theleftmost barrier 543 are pushed into the adjacent first region 541 a asindicated by arrow R8. In this manner, the polarization-induced fieldsact to funnel electrons and holes into the adjacent quantum wells 542,where they may combine efficiently and emit light (or other radiation)at the target wavelength.

FIGS. 7A and 7B illustrate the results of a numerical simulationconducted to demonstrate the efficacy of structures havingcharacteristics generally similar to those described above withreference to FIGS. 2-6. In this particular simulation, the structuresinclude a stack of nine quantum wells 542, each having a width ofapproximately 3 nanometers and a composition of Ga_(0.8)In_(0.2)N.Neighboring quantum wells 542 are separated by barriers 543 having awidth of approximately 100 nanometers and a composition ofGa_(0.9)In_(0.1)N. The outermost quantum wells are positioned adjacentto layers of gallium nitride 541 a, 541 b. The silicon dopingconcentration is 5×10¹⁸ cm⁻³ in the gallium nitride 541 a, 541 b and1×10¹⁸ cm⁻³ in the barriers 543. Non-radiative recombination lifetimesin the barriers are assumed to be 5 nanoseconds for both electrons andholes. FIG. 7A illustrates a simulation for which the polarizationwithin the structure is zero. This case is analogous to potentialwavelength conversion structures with non-polar semiconductors, or withpolar semiconductors, but along non-polar directions. FIG. 7Billustrates the results when the polarization within the structure isthe predicted amount for c-plane gallium indium nitrideheterostructures. In both simulations, all layers are assumed to bepseudomorphic (e.g., strained).

The simulation method solves the Poisson equation and continuityequations for electrons and holes self-consistently. The boundarycondition for the simulation is that no current flows in or out of thestructure, which is the case if the structure is not connected to anelectrical circuit. An undepleted optical pump is assumed to generateelectron-hole pairs uniformly within both the quantum wells 542 and thebarriers 543. This pump corresponds to an LED or other light source thatgenerates the optical energy absorbed by the radiative structure andreemitted at a different wavelength. The total number of generatedcarriers is assumed to be equal for the results shown in both FIGS. 7Aand 7B, and the carriers are permitted to move freely through drift anddiffusion processes and recombine either non-radiatively (throughShockley-Reed-Hall recombination or Auger recombination) or radiatively,thereby generating light.

Still referring to FIGS. 7A and 7B, the steady state band diagrams ofthe two structures in the presence of optical exitation are shown. Theelectron and hole quasi-Fermi levels are separated, as expected. Thedifferences between the two cases are clearly shown. In FIG. 7B, thepolarization produces electric fields that exist in the barriers 543 andthat act to direct carriers toward the quantum wells 542 where theyaccumulate to sufficiently high densities to recombine efficiently. Thisis not the case for the structure without polarization (shown in FIG.7A), where the lifetime of the carriers generated in the barriers 543will be longer.

The ultimate output of the calculation based on the foregoingsimulations is the relative rate of radiative and non-radiativerecombination. In the structure represented by FIG. 7B, theShockley-Reed-Hall recombination rate was reduced by 30.9%, while theradiative rate was increased by 17.6%. Accordingly, FIGS. 7A and 7Bdemonstrate the efficiency enhancement that maybe obtained by takingadvantage of polarization-enhanced carrier capture.

One feature of at least some of the foregoing embodiments describedabove with reference to FIGS. 2-7B is that the disclosed technology caninclude structures that are selected based on crystal-based materialpolarization to concentrate carriers in regions selected to emitradiation at particular wavelengths. These structures can moreefficiently convert energy received at one wavelength to energy emittedat a second wavelength. An advantage of this arrangement is that it canreduce power consumed by the device, and/or increase the light output bythe device, when compared with conventional wavelength converters, forexample, phosphor coatings and the like. In particular, the radiativestructure can be manufactured and operated with a phosphor.

From the foregoing, it will be appreciated that specific embodiments ofthe disclosed technology have been described herein for purposes ofillustration, but that various modifications may be made withoutdeviating from the disclosed technology. For example, materials havingdifferent material polarizations can be combined based on isotropicand/or anisotropic material polarizations. The material polarizations ofadjacent elements in a wavelength converter structure can changeabruptly at the hetero interfaces, as was generally shown above, orgradually, e.g., by gradually varying the concentration of indium at thehetero interfaces between GaInN barriers and quantum wells. Certainembodiments of the technology were described in the context ofparticular materials (e.g., gallium nitride and gallium indium nitride),but in other embodiments, other materials can be used to produce similarresults. Certain embodiments of the technology were described above inthe context of shifting the wavelength of visible light. In otherembodiments similar structures and methods can be used to shift energyat other wavelengths.

Certain aspects of the technology described in the context of particularembodiments may be combined or eliminated in other embodiments. Forexample, particular embodiments can include more or fewer barriers andquantum wells than described above with reference to FIGS. 5-7B. Thewavelength conversion structure can include a single first region ratherthan two, e.g., if the first region is properly doped. The structuresdescribed above can be combined with additional structures (e.g.,lenses, power sources controllers, and/or other devices) depending uponthe functions the structures are intended to perform. Further, whileadvantages associated with certain embodiments have been described inthe context of those embodiments, other embodiments may also exhibitsuch advantages, and not all embodiments need necessarily exhibit suchadvantages to fall within the scope of the present technology.Accordingly, the present disclosure and associated technology canencompass other embodiments not expressly shown or described herein.

I/we claim:
 1. A solid state lighting (SSL) device, comprising: a lightemitting diode configured to emit light at a first wavelength; and asolid state radiative semiconductor structure having a first regioncoupled to the light emitting diode, and a second region adjacent to thefirst region, wherein— the first region is positioned to absorb thelight at the first wavelength and thereby generate electrons, holes, orboth electrons and holes in the first region, the first region having afirst material polarization, and the second region has a second materialpolarization different than the first material polarization, wherein thefirst and second material polarizations form a potential gradient havingan orientation at an interface between the first and second regionsselected to drive the generated electrons, holes, or both electrons andholes in the radiative structure from the first region to the secondregion such that they radiatively recombine at the second region to emitlight at a second wavelength different than the first wavelength.
 2. TheSSL device of claim 1 wherein the first region is an alloy of galliumnitride, and the second region is an alloy of gallium nitride differentthan the alloy of the first region.
 3. The SSL device of claim 2 whereinthe second region includes gallium indium nitride.
 4. The SSL device ofclaim 2 wherein the first region includes gallium indium nitride with afirst concentration of indium that is greater than zero, and the secondregion includes gallium indium nitride with a second concentration ofindium greater than the first concentration.
 5. The SSL device of claim1 wherein a composition of the radiative structure has a step change atthe interface.
 6. The SSL device of claim 1 wherein the first and secondregions are polar regions, and wherein the interface is at a c-plane ofat least one of the first and second regions.
 7. The SSL device of claim1 wherein the first region includes a barrier and the second regionincludes a quantum well, and wherein the SSL device includes a stack ofalternating barriers and quantum wells, with individual barrierscomprising indium gallium nitride with a first concentration of indium,and with individual quantum well comprising indium gallium nitride witha second concentration of indium greater than the first.
 8. The SSLdevice of claim 7 wherein the quantum wells and the barriers have thesame wurtzite crystal orientation.
 9. The SSL device of claims 1 whereinthe light emitting diode includes an active region positioned adjacentto an N-type material and a P-type material, the active region beingpositioned to emit the light at the first wavelength directed to theradiative structure.
 10. The SSL device of claim 1 wherein the first andsecond regions include a material having a common constituent, whereinthe constituent has a first concentration in the first material and asecond concentration different than the first concentration in thesecond material.
 11. A method for forming a solid state lighting device,comprising: selecting a radiative structure having a first region and asecond region, the first region having a first value of a materialcharacteristic, the second region having a second value of the materialcharacteristic, with the first and second values producing a potentialgradient to drive at least one of electrons and holes in the radiativestructure from the first region to the second region; and positioningthe radiative structure with the first region proximate to a radiationsource to absorb radiation at a first wavelength, and with the secondregion positioned to receive energy from the first region and emit theenergy at a second wavelength via radiative recombination, wherein thesecond wavelength is different than the first wavelength, wherein thematerial characteristic includes material polarization, and whereinselecting a radiative structure includes selecting a radiative structurewith the first region having a first material polarization value and thesecond region having a second material polarization value different thanthe first material polarization value, with the differences between thefirst and second material polarization values forming an electric fieldat an interface between the first and second regions.
 12. The method ofclaim 11 wherein the radiation source include a light emitting diode.13. The method of claim 11, further comprising disposing the first andsecond materials adjacent to and fixed relative to each other.
 14. Themethod of claim 11 wherein selecting the first material includesselecting the first material to include gallium nitride, and whereinselecting the second material includes selecting the second material toinclude gallium indium nitride.
 15. The method of claim 11 whereinselecting the first material includes selecting the first material toinclude gallium indium nitride with a first concentration of indium, andwherein selecting the second material includes selecting the secondmaterial to include gallium indium nitride with a second concentrationof indium different than the first concentration.
 16. The method ofclaim 11, further comprising selecting both the first and secondmaterials to have wurtzite crystal structures.
 17. The method of claim11, further comprising selecting both the first and second materials tohave the same crystal orientation.
 18. A method for operating aradiative structure, comprising: receiving radiation of a firstwavelength at a first region of a solid state radiative semiconductorstructure, the first region having a first material polarization and afirst density of electrons and holes; transmitting electrons from thefirst region to a second region of the solid state radiativesemiconductor structure to increase a density of the electrons at thesecond region to a second density higher than the first density, thesecond region having a second material polarization different than thefirst material polarization; and combining electrons with holes at thesecond region to radiate energy at a second wavelength different thanthe first wavelength.
 19. The method of claim 18 wherein the firstregion is one of a plurality of first regions, and the second region isone of a plurality of second regions, and wherein individual secondregions are positioned between consecutive first regions, and whereintransmitting electrons includes transmitting electrons from a firstregion on one side of the second region, and wherein the method furthercomprises transmitting holes to the second region from a first region onan opposite side of the second region.
 20. The method of claim 18wherein radiating energy at the second wavelength includes radiatingenergy at the second wavelength in the visible spectrum, withoutradiating the energy from a phosphor.